Another here using the S&S Hiliner, DD is 5 and 19 kgs and we have found that although I was nervous about it, it fits her super well, even when she has fallen asleep it fits really well and looks safe and comfortable. I also read about the H-harness being no safer than a lap belt at this age and weight and actually LESS safe if it's fitted incorrectly and sits incorrectly which a lot of the time they are because they are so tricky to do up. I would be possibly emailing the companies whose seats you are looking at and see what they recommend seeing as he is near the upper weight limit for them all. xoxox